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Anaerobic Digestate As a Nutrient Medium for the Growth of the Green Microalga Neochloris Oleoabundans

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Anaerobic Digestate As a Nutrient Medium for the Growth of the Green Microalga Neochloris Oleoabundans Environ. Eng. Res. 2016; 21(3): 265-275 pISSN 1226-1025 http://dx.doi.org/10.4491/eer.2016.005 eISSN 2005-968X Anaerobic digestate as a nutrient medium for the growth of the green microalga Neochloris oleoabundans Husam A. Abu Hajar1†, R. Guy Riefler1, Ben J. Stuart2 1Department of Civil Engineering, Ohio University, Athens, OH 45701, USA 2Department of Civil & Environmental Engineering, Old Dominion University, Norfolk, VA 23529, USA ABSTRACT In this study, the microalga Neochloris oleoabundans was cultivated in a sustainable manner using diluted anaerobic digestate to produce biomass as a potential biofuel feedstock. Prior to microalgae cultivation, the anaerobic digestate was characterized and several pretreatment methods including hydrogen peroxide treatment, filtration, and supernatant extraction were investigated and their impact on the removal of suspended solids as well as other organic and inorganic matter was evaluated. It was found that the supernatant extraction was the most convenient pretreatment method and was used afterwards to prepare the nutrient media for microalgae cultivation. A bench-scale experiment was conducted using multiple dilutions of the supernatant and filtered anaerobic digestate in 16 mm round glass vials. The results indicated that the highest growth of the microalga N. oleoabundans was achieved with a total nitrogen concentration of 100 mg N/L in the 2.29% diluted supernatant in comparison to the filtered digestate and other dilutions. Keywords: Anaerobic digestion, Biofuels, Microalgae, Neochloris oleoabundans 1. Introduction able in nature with lower harmful emissions such as CO, hydro- carbons, and particulate matter, and no SOx emissions, besides the potential of utilizing the carbon dioxide portion of the flue The need for unconventional fuel feedstocks such as biofuels emerg- gas from power plants as a carbon source for the growth of micro- es due to the environmental consequences of utilizing conventional algae [1, 6, 8]. fossil fuels such as the gaseous emissions [1-3]. Biodiesel is a Neochloris oleoabundans is a freshwater and saline media micro- biofuel that is typically produced from oleaginous crops via the alga from the Chlorophyceae class and the Chlorococcaceae family transesterification of their oils with methanol or ethanol to produce [1, 10]. Li et al. [6] pointed out that this microalga is promising; fatty acid methyl esters (FAME). These crops include rapeseed, as the lipid productivity of this species was nearly twice that soybean, sunflower, and palm [2-4]. of any other microalga studied for the purpose of biodiesel Microalgae are eukaryotic aquatic photosynthetic micro- organisms capable of harvesting the solar energy effectively to production. Moreover, the majority of the fatty acids produced produce biomass that is ideal for the production of biodiesel [2, by this microalga are saturated fatty acids in the range of 16-20 5-7]. Microalgae, similar to plants, convert solar energy to chemical carbons, which makes this microalga ideal for biodiesel synthesis, energy via photosynthesis, producing organic biomass from carbon even though most of the previous studies reported utilizing this dioxide and water. Approximately, 1.83 kg carbon dioxide is fixed species as a feeding source for aquaculture species such as mussels for each 1 kg biomass produced [8]. There are many advantages [6]. The highest biomass concentrations reported in the literature to producing biofuels from microalgae such as the high areal yield, for the phototrophic cultivation of this microalga were in the 2-5.17 along with avoiding competition for fertile soil; since microalgae g/L range [6, 11-15]. can grow on non-arable lands, using non-potable water, in a con- A major challenge to the production of biodiesel from microalgae tinuous rather than seasonal mode with a significantly lower water is the relatively high capital and operational costs compared to consumption rate and in an aqueous suspension system that pro- conventional fossil fuels due to cultivation requirements [5, 9]. vides more access to nutrients and water [1-4, 8-9]. Additionally, Nutrients availability, mainly nitrogen and phosphorus, is a key biodiesel derived from microalgal lipids is sustainable and renew- factor for the growth of microalgae, which adds to the production This is an Open Access article distributed under the terms Received January 12, 2016 Accepted April 9, 2016 of the Creative Commons Attribution Non-Commercial License † (http://creativecommons. org/ licenses/by-nc/3.0/) which per- Corresponding author mits unrestricted non-commercial use, distribution, and reproduction in any Email: [email protected] medium, provided the original work is properly cited. Tel: +1-962-6-5355000 Fax: +1-962-6-5355522 Copyright © 2016 Korean Society of Environmental Engineers 265 Husam A. Abu Hajar et al. cost. Industrial, municipal, and agricultural wastewaters may pro- suspended solids as well as the interference from other vide the necessary nutrients for the growth, where microalgae microorganisms. According to Levine et al. [3], there was no sig- remove nitrogen and phosphorus from wastewater via direct nificant difference between raw and autoclaved diluted effluent uptake. Moreover, the oxygen produced by microalgae can be uti- on the microalgae biomass productivity. It was thought initially lized by the aerobic bacteria for further reduction in the organic that the native bacteria within the effluent produce vitamin B12, matter [3, 7, 16]. Microalgae such as Chlamydomonas, Botryococcus, which is necessary for the growth of microalgae. Additionally, Chlorella, Haematococcus, Spirulina, and Scenedesmus have been the heterotrophic microorganisms may contribute to the growth utilized for wastewater treatment [16]. For instance, Tam and Wong of microalgae by remineralizing nutrients and producing carbon [17] reported that the microalga Chlorella pyrenoidosa grew well dioxide as a result of organic carbon oxidation [3]. On the other when cultivated using supernatant from the preliminary and pri- hand, centrifugation is an energy intensive process, which may mary sedimentation and the secondary effluent from an activated limit its applicability as a pretreatment step to the anaerobic diges- sludge process. In addition, Choi and Lee [16] stated that nitrogen tate [23]. and phosphorus removal efficiencies of 81-85% and 32-36%, re- In this study, the AD from two sources was characterized in spectively, were achieved when the microalga Chlorella vulgaris order to be used as a nutrient source for microalgae. Several pretreat- was cultivated with wastewater from the preliminary sed- ment methods other than those discussed in the previous studies imentation of a sewage plant. However, growth inhibition caused were explored including filtration with variable mesh sizes, hydrogen by elevated concentrations of ammonia, urea, and volatile fatty peroxide oxidation, and supernatant extraction. The purpose of these acids may limit the use of microalgae as means of secondary waste- treatment methods was to reduce the suspended solids in the nutrient water treatment [18, 19]. medium as well as eliminate the potential toxicity caused by elevated Anaerobic digestion of animal manure is an often-used approach concentrations of organic and inorganic matter. Further, the growth to reduce the biological oxygen demand in the waste. However, of the microalga N. oleoabundans using diluted AD was evaluated nutrients are not eliminated via this route; in fact they become by a bench-scale experiment using 16 mm round glass vials. This more bioavailable in the forms of ammonium and phosphate. Using experiment covered unfiltered and filtered digestate as well as a the anaerobic digestate (AD) to cultivate microalgae has some chal- wider range of dilutions compared to the previous studies in literature. lenges including the potential high and unbalanced concentrations of nutrients, turbidity, other competing microorganisms, as well as the potential toxicity cause by elevated COD and ammonia 2. Materials and Methods concentrations; therefore, diluting the AD may become necessary before microalgae inoculation [3, 7]. Furthermore, pretreatment 2.1. Anaerobic Digestate methods such as autoclaving, filtration, or other techniques are AD was sampled from two sources in Columbus, OH. Source A often applied to wastewaters in general and AD in particular prior is a commercial digester where animal manure and other organic to microalgae cultivation in order to reduce the suspended solids wastes are digested anaerobically to produce biogas, while source concentration as well as prevent interference from other micro- B is an anaerobic digester processing waste activated sludge from organisms such as bacteria or protozoa [16, 20]. a domestic wastewater treatment plant. Samples were brought Several studies indicated the potential of growing the microalga to the laboratory and stored at 4˚C until the time of analysis. N. oleoabundans using the anaerobic digestion effluent or digestate [3, 21, 22]. However, the range of dilutions covered by each in- 2.2. Analytical Methods dividual study was narrow. For instance, Levine et al. [3] inoculated N. oleoabundans in 50-, 100-, and 200-fold diluted anaerobic diges- Measurements of total solids (TS), volatile solids (VS), total sus- tion effluent under 200 μmol/m2/s light intensity. They concluded pended solids (TSS), volatile suspended solids (VSS) were per- that 50-fold dilution which was equivalent to 60 mg N/L total formed according
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